<!DOCTYPE html>
<html lang="en">
<head>          
    <meta charset="UTF-8">
    <title></title>
    
    <style>
        body{
            margin:0 auto;
        }
    </style>

</head>
<body>
    <div style="height:2000px;"></div>
        <input type="button" value="提交数据"/>

        <div style="z-index:1;opacity:0.5;position:fixed;left:0;right:0;top:0;bottom:0;">
            <img src="login.jpg">
        </div>
        <div style="z-index:2;position:fixed;left:50%;top:50%;margin-left:-50px;margin-top:-50px;">
            <div style="width:100px;height:100px;">
                <input />
                <input />
                <input />
                <input />
            </div>
    </div>

</body>
</html>